They are fairly easy, and inexpensive. I had a 12 ball one in my old house, and it was totally enclosed, kept the dust off...LOL But now with the disability, I am down to 5 balls, and really only use 2.
Three in a triple roller and two in a double bag, next to my spinner cabinet.
Have fun making it and enjoy it.
Gene, again, invest in a spinner....well worth it....just ask charlest!